This invention is directed to breathable films and laminates containing them. Most of the moisture vapor breathability is induced by stretching the films by a small amount in the cross direction.
Laminates which are breathable to water vapor but substantially impermeable to liquid water are known in the art, and are commonly used in diaper backings, other personal care absorbent garments, medical and industrial protective garments, and the like. These laminates may be composed of a breathable, stretch-thinned filled film and a spunbond web. The breathable film can be formed by blending one or more polyolefins with an inorganic particulate filler, forming a film from the mixture, and stretching the film to cause void formation around the filler particles. The resulting film may have thin polymer membranes around the filler particles which permit molecular diffusion of water vapor, while the overall film substantially blocks transmission of liquid water, or may have micropores going through the film. The breathable film can be laminated to a nonwoven web, for instance, a spunbond web, by thermal or adhesive bonding. The spunbond web adds abrasion resistance, strength and integrity to the breathable laminate, and provides a soft, cloth-like feel.
One trend affecting the personal care absorbent garment industry, and the protective garment industry, involves the demand and need for products with higher breathability to water vapor, which retain or increase the barrier to water, blood and other liquid substances. This trend reflects the demand for increased wearer comfort without loss of barrier performance. Another trend affecting these industries involves the demand and need for products having better fit, which conform to the contours of the wearer""s body.
Still another trend involves the demand and need for products which are less expensive to produce, and which use less materials without sacrificing desirable product characteristics. Still another trend involves the demand and need for laminates having higher breathability to moisture vapor in selected regions of the laminates. In diapers and other pant-like absorbent articles, liquid can accumulate in the crotch region. When this happens, heat from the wearer""s body can cause the space between garment and the wearer to become saturated with water vapor, facilitating the occurrence of diaper rashes and other skin irritations. The best way to effectively vent the water vapor is through other regions of the garment which are not affected by the pool of liquid in the crotch.
The present invention is directed to a breathable film, and a breathable laminate including the film and at least one nonwoven web. The film has a first state in which it has not been extended in the cross-direction, and a second state in which it has been extended by 25% in the cross-direction. The film has a first water vapor transmission rate (WVTR) of at least 500 grams/m2-24 hours in the first state, and a second WVTR in the second state, determined from the WVTR test procedure described below. The second WVTR in the second state is at least about 225% of the first WVTR, and is not less than about 4000 grams/m2-24 hours. The large increase in WVTR between the first state and the second state occurs solely as a result of stretching the film by about 25% in the cross-direction.
The present invention is also directed to a breathable laminate which exhibits similar properties. The nonwoven web is selected, and is bonded to the breathable film, so as not to substantially impair the breathability of the film. In essence, the breathability of the laminate is determined by the breathability of the film, although the WVTR values may be somewhat lower for the laminate depending on the bonding technique employed. The laminate has a first state in which it has not been extended in the cross-direction, and a second state in which the laminate (including the film) has been extended by about 25% in the cross-direction of the film. The laminate has a first WVTR in the first state which is at least 500 grams/m2-24 hours, determined from the WVTR test procedure described below. The laminate has a second WVTR in the second state which is at least 225% of the first WVTR, and is not less than about 4000 grams/m2-24 hours.
The breathable laminate can be used in a wide variety of personal care absorbent articles and protective garments. In one embodiment, the laminate is used as a backsheet in a disposable diaper or other pant-like absorbent garment. The diaper or other pant-like garment is initially undersized, representing a material savings. To don the garment on a wearer, the front and back regions in the garment (including the laminate) are stretched by about 25% of the original width of the laminate, in the cross-direction of the film. This stretching causes the front and back regions to have substantially higher WVTR than the crotch region, which is not significantly stretched during donning.
With the foregoing in mind, it is a feature and advantage of the invention to provide a breathable film, and a corresponding film/nonwoven web laminate, to which high moisture vapor breathability can be induced by only minor stretching in the cross-direction of the film.
It is also a feature and advantage of the invention to provide a garment, such as a pant-like absorbent garment, to which selected regions of high breathability can be induced by minor stretching occurring during donning of the garment.
The foregoing and other features and advantages of the invention will become further apparent from the following detailed description of the presently preferred embodiments.
The term xe2x80x9cextendiblexe2x80x9d is used herein to mean a material which upon application of a stretching force, can be extended in a particular direction (e.g., the cross-direction), to a stretched dimension (e.g., width) which is at least 25% greater than an original, unstretched dimension. When the stretching force is removed after a one-minute holding period, the material preferably does not retract, or retracts by not more than 30% of the difference between the stretched dimension and the original dimension. Thus, a material having a width of one meter, which is extendible in the cross direction, can be stretched to a width of at least 1.25 meters. When the stretching force is released, after holding the extended width for one minute, a material stretched to a width of 1.25 meters will preferably not retract, or will retract to a width of not less than 1.175 meters. Extendible materials are different from elastic materials, the latter tending to retract most of the way to their original dimension when a stretching force is released. The stretching force can be any force sufficient to extend the material to between 125% of its original dimension, and its maximum stretched dimension in the selected direction (e.g., the cross direction) without rupturing it.
The xe2x80x9cpercent retractionxe2x80x9d is determined when an extended material is relaxed to where the retractive force drops below 10 grams for a 3-inch wide sample. xe2x80x9cPercent permanent setxe2x80x9d is 100 minus xe2x80x9cpercent retraction.xe2x80x9d
The term xe2x80x9cinelasticxe2x80x9d refers both to materials that do not stretch by 25% or more and to materials that stretch by that amount but do not retract by more than 30%. Inelastic materials include extendible materials, as defined above, as well as materials that do not extend, e.g., which tear when subjected to a stretching force.
The term xe2x80x9cmachine directionxe2x80x9d as applied to a nonwoven web, refers to the direction of travel of a conveyor passing beneath the spinnerette or similar extrusion or forming apparatus for the filaments, which causes the filaments to have primary orientation in the same direction. While the filaments may appear wavy, or even randomly oriented in a localized section of a nonwoven web, they usually have an overall machine direction of orientation which was parallel to the movement of the conveyor that carried them away from the extrusion or forming apparatus.
The term xe2x80x9cmachine directionxe2x80x9d as applied to a film, refers to the direction on the film that was parallel to the direction of travel of the film as it left the extrusion or forming apparatus. If the film passed between nip rollers or chill rollers, for instance, the machine direction is the direction on the film that was parallel to the surface movement of the rollers when in contact with the film.
The term xe2x80x9cmachine directionxe2x80x9d as applied to a laminate including at least one film and at least one nonwoven web, refers to the machine direction of the film component of the laminate.
The term xe2x80x9ccross directionxe2x80x9d for a nonwoven web, film, or laminate refers to the direction perpendicular to the machine direction. Dimensions measured in the cross direction are referred to as xe2x80x9cwidthxe2x80x9d dimensions, while dimensions measured in the machine direction are referred to as xe2x80x9clengthxe2x80x9d dimensions.
The terms xe2x80x9cbreathable film,xe2x80x9d xe2x80x9cbreathable laminatexe2x80x9d or xe2x80x9cbreathable outer cover materialxe2x80x9d refer to a film, laminate, or outer cover material having a water vapor transmission rate (xe2x80x9cWVTRxe2x80x9d) of at least about 500 grams/m2-24 hours, using the WVTR Test Procedure described herein. The term xe2x80x9chigher breathabilityxe2x80x9d simply means that a second material has a higher WVTR than a first material. Breathable materials typically rely on molecular diffusion of vapor, or vapor passage through micropores, and are substantially liquid impermeable.
The term xe2x80x9cliquid water-permeable materialxe2x80x9d refers to a material present in one or more layers, such as a nonwoven fabric, which is porous, and which is liquid water permeable due to the flow of water and other aqueous liquids through the pores. The spaces between fibers or filaments in a nonwoven web can be large enough and frequent enough to permit leakage and flow of liquid water through the material.
The term xe2x80x9cnonwoven fabric or webxe2x80x9d means a web having a structure of individual fibers or threads which are interlaid, but not in a regular or identifiable manner as in a knitted fabric. Nonwoven fabrics or webs have been formed from many processes such as, for example, meltblowing processes, spunbonding processes, air laying processes, coforming processes, and bonded carded web processes. The basis weight of nonwoven fabrics is usually expressed in ounces of material per square yard (osy) or grams per square meter (gsm) and the fiber diameters useful are usually expressed in microns. (Note that to convert from osy to gsm, multiply osy by 33.91.)
The term xe2x80x9cmicrofibersxe2x80x9d means small diameter fibers typically having an average fiber denier of about 0.005-10. Fiber denier is defined as grams per 9000 meters of a fiber. For a fiber having circular cross-section, denier may be calculated as fiber diameter in microns squared, multiplied by the density in grams/cc, multiplied by 0.00707. For fibers made of the same polymer, a lower denier indicates a finer fiber and a higher denier indicates a thicker or heavier fiber. For example, the diameter of a polypropylene fiber given as 15 microns may be converted to denier by squaring, multiplying the result by 0.89 g/cc and multiplying by 0.00707. Thus, a 15 micron polypropylene fiber has a denier of about 1.42 calculated as (152xc3x970.89xc3x970.00707=1.415). Outside the United States the unit of measurement is more commonly the xe2x80x9ctex,xe2x80x9d which is defined as the grams per kilometer of fiber. Tex may be calculated as denier/9.
The term xe2x80x9cspunbonded fibersxe2x80x9d refers to small diameter fibers which are formed by extruding molten thermoplastic material as filaments from a plurality of fine capillaries of a spinnerette having a circular or other configuration, with the diameter of the extruded filaments then being rapidly reduced as by, for example, in U.S. Pat. No. 4,340,563 to Appel et al., and U.S. Pat. No. 3,692,618 to Dorschner et al., U.S. Pat. No. 3,802,817 to Matsuki et al., U.S. Pat. Nos. 3,338,992 and 3,341,394 to Kinney, U.S. Pat. No. 3,502,763 to Hartmann, U.S. Pat. No. 3,502,538 to Petersen, and U.S. Pat. No. 3,542,615 to Dobo et al., each of which is incorporated herein in its entirety by reference. Spunbond fibers are quenched and generally not tacky when they are deposited onto a collecting surface. Spunbond fibers are generally continuous and often have average deniers larger than about 0.3, more particularly, between about 0.6 and 10.
The term xe2x80x9cmeltblown fibersxe2x80x9d means fibers formed by extruding a molten thermoplastic material through a plurality of fine, usually circular, die capillaries as molten threads or filaments into converging high velocity heated gas (e.g., air) streams which attenuate the filaments of molten thermoplastic material to reduce their diameter, which may be to microfiber diameter. Thereafter, the meltblown fibers are carried by the high velocity gas stream and are deposited on a collecting surface to form a web of randomly dispersed meltblown fibers. Such a process is disclosed for example, in U.S. Pat. No. 3,849,241 to Butin et al. Meltblown fibers are microfibers which may be continuous or discontinuous, are generally smaller than about 1.0 denier, and are generally self bonding when deposited onto a collecting surface.
The term xe2x80x9cfilmxe2x80x9d refers to a thermoplastic film made using a film extrusion process, such as a cast film or blown film extrusion process. This term includes films rendered microporous by mixing polymer with filler, forming a film from the mixture, and stretching the film.
The term xe2x80x9cmicroporousxe2x80x9d refers to films having voids separated by thin polymer membranes and films having micropores passing through the films. The voids or micropores can be formed when a mixture of polymer and filler is extruded into a film and the film is stretched, preferably uniaxially in the machine direction. Microporous films tend to have water vapor transmission due to molecular diffusion of water vapor through the membranes or micropores, but substantially block the passage of aqueous liquids.
The term xe2x80x9cpolymerxe2x80x9d includes, but is not limited to, homopolymers, copolymers, such as for example, block, graft, random and alternating copolymers, terpolymers, etc. and blends and modifications thereof. Furthermore, unless otherwise specifically limited, the term xe2x80x9cpolymerxe2x80x9d shall include all possible geometrical configurations of the material. These configurations include, but are not limited to isotactic, syndiotactic and atactic symmetries.
The term xe2x80x9cgarmentxe2x80x9d includes pant-like absorbent garments and medical and industrial protective garments. The term xe2x80x9cpant-like absorbent garmentxe2x80x9d includes without limitation diapers, training pants, swim wear, absorbent underpants, baby wipes, adult incontinence products, and feminine hygiene products.
The term xe2x80x9cmedical protective garmentxe2x80x9d includes without limitation surgical garments, gowns, aprons, face masks, and drapes. The term xe2x80x9cindustrial protective garmentxe2x80x9d includes without limitation protective uniforms and workwear.
The term xe2x80x9cneckxe2x80x9d or xe2x80x9cneck stretchxe2x80x9d interchangeably means that the fabric, nonwoven web or laminate is drawn such that it is extended under conditions reducing its width or its transverse dimension by stretching lengthwise or increasing the length of the fabric. The controlled drawing may take place under cool temperatures, room temperature or greater temperatures and is limited to an increase in overall dimension in the direction being drawn up to the elongation required to break the fabric, nonwoven web or laminate, which in most cases is about 1.2 to 1.6 times. When relaxed, the fabric, nonwoven web or laminate does not return totally to its original dimensions. The necking process typically involves unwinding a sheet from a supply roll and passing it through a brake nip roll assembly driven at a given linear speed. A take-up roll or nip, operating at a linear speed higher than the brake nip roll, draws the fabric and generates the tension needed to elongate and neck the fabric. U.S. Pat. No. 4,965,122 issued to Morman, and commonly assigned to the assignee of the present invention, discloses a reversibly necked nonwoven material which may be formed by necking the material, then heating the necked material, followed by cooling and is incorporated herein by reference in its entirety. The heating of the necked material causes additional crystallization of the polymer giving it a partial heat set. If the necked material is a spunbond web, some of the fibers in the web may become crimped during the necking process, as explained in U.S. Pat. No. 4,965,122.
The term xe2x80x9cneckable materialxe2x80x9d or xe2x80x9cneckable layerxe2x80x9d means any material or layer which can be necked such as a nonwoven, woven, or knitted material, or a laminate containing one of them. As used herein, the term xe2x80x9cnecked materialxe2x80x9d refers to any material which has been drawn in at least one dimension, (e.g. lengthwise), reducing the transverse dimension, (e.g. width), such that when the drawing force is removed, the material can be pulled back to its original width. The necked material generally has a higher basis weight per unit area than the un-necked material. When the necked material is pulled back to its original width, it should have about the same basis weight as the un-necked material. This differs from stretching/orienting the film layer, during which the film is thinned and the basis weight is reduced. Preferred nonwoven webs for use in the invention are made from an inelastic polymer.
The term xe2x80x9cpercent neckdownxe2x80x9d refers to the ratio determined by measuring the difference between the un-necked dimension and the necked dimension of the neckable material and then dividing that difference by the un-necked dimension of the neckable material.